This invention relates to methods and systems for communicating high-speed data and streaming content (video and audio, both xe2x80x9clivexe2x80x9d and pre-recorded) to mobile users with small, low-profile antennas using satellite-based communication systems.
The success of the worldwide direct broadcast satellite service (Ku BSS)xe2x80x94also called direct-to-home (xe2x80x9cDTHxe2x80x9d)xe2x80x94has resulted in hundreds of video and audio channels being available to customers regardless of their location relative to high-speed terrestrial lines (DSL, cable). Customer acceptance has been good even in urban areas due in large measure to the very small (xcx9c45 cm) antenna reflector (xe2x80x9cdishxe2x80x9d) required. The small antenna size minimizes wind loading and makes roof mounting feasible. This small antenna size results from the high ERP (xcx9c53 dBW) generated by the satellite payload, and the 9-degree satellite spacing in the Ku BSS bands imposed by regulatory agencies (such as the FCC). For US BSS service, the satellites radiate a circularly polarized wave simplifying the installation process. As long as the ground terminal antenna is properly xe2x80x9cpointedxe2x80x9d at the satellite, the antenna (or feed) can be rotated around the xe2x80x9cpointingxe2x80x9d axis without affecting performance. This is particularly desirable for moving platforms. The uplink signal is provided to each transponder independently, and merely xe2x80x9crepeatedxe2x80x9d at an amplified level on the downlink. DTH satellites have also been used to provide high-speed data two-way data by coupling a xe2x80x9cforward-linkxe2x80x9d using a receive-only satellite terminal to a separate xe2x80x9creturn-linkxe2x80x9d (such as a xe2x80x9cdial-upxe2x80x9d modem, cellular/PCS modem, or a lower-speed two way satellite).
Despite the desirable features of the DTH systems, there are only a very limited number of orbital slots available because of the 9-degree spacing. Over the continental US (xe2x80x9cCONUSxe2x80x9d), only three centrally located slots are available and all three slots are occupied, leaving no capacity for additional services.
Many of the desirable qualities of the broadcast DTH service also appear in the Ku FSS bands. The 2-degree spacing of the FSS satellites creates many more locations for satellite broadcast. One Canadian DTH service provider, for example, has used Ku FSS satellites for broadcast to the home with larger dishes due to the lower ERP of Ku FSS satellites (typically 47-50 dBW). A typical high-speed two-way Ku FSS terminal in the U.S. market has a reflector close to 1 m in diameter.
For a CONUS Ku FSS mobile service, there are additional complications in the ground terminal implementation compared to the reception of a circularly polarized DTH signal:
(1) Due to the variation of polarization with movement of the platform, a dual polarized ground antenna is required in all cases (even if only one transponder is being received), or the polarization must be xe2x80x9ctrackedxe2x80x9d electronically or with an additional third-axis mechanical tracking loop.
(2) The much smaller beamwidth of the receive antenna compared to the DTH application (1.9 degrees for a 1 m antenna versus 3.5 degrees for a 45 cm antenna) makes tracking and alignment more critical. Since components like gyros are used to provide inertial pointing references, the improved accuracy; has a dramatic impact on component costs for the positioner. A 1 degree pointing error (representative of mechanical pointing accuracy which can be readily achieved with inexpensive positioners) induces a xcx9c3 dB gain loss for the 1 m antenna, but only xcx9c1 dB for the 45 cm antenna.
(3) The lower radiated power of Ku FSS satellite transponders (typically ERP in the 47-50 dBW range compared to the higher 53-56 dBW of the DTH satellites) requires the use of a larger ground terminal antenna (xcx9c2xc3x97diameter, or 6 dB higher gain) to ensures that the received signal strength is adequate.
Although there is significant interest in the use of communications satellites for mobile applications, there has been very limited deployment of such services based on Ku FSS satellites. Interest remains high for such communications services, as shown by the following issues in Table I:
(1) Satellites provide the only ubiquitous coverage of large areas such as the continental US (xe2x80x9cCONUSxe2x80x9d) or Europe, and for broadcast applications they are extremely efficient from a cost viewpoint;
(2) For mobile applications, they remain the only current technology capable of providing high-speed data to a rapidly moving platform regardless of location;
(3) Since Ku FSS satellites do not generally use numerous narrow downlink spot beams, there is no complicated procedure at the control center for handing off between beams due to the motion of the user; and
(4) Satellites continue to remain the only viable technology for providing broadcast and high-speed data over the oceans. The use of Ku FSS satellites for two-way, high-speed communications has historically required large, expensive tracking antennas for both pointing the beam and continuously adjusting the polarization due to movement.
In view of the foregoing, there is a significant need in the marketplace for a solution that combines the small size of the DTH antenna with the simpler tracking requirements associated with the reception of circular polarization.
The present invention provides a satellite-based communications system and method for supporting improved high speed data (including video streaming) communications to mobile terminal or units having small, low profile antennas suitable for operation on an aircraft fuselage or on the roof of land mobile platforms, such as SUV""s and minivans. The method is typically compatible with geostationary satellites operating in the xe2x80x9cFSSxe2x80x9d frequency bands, as well as new satellites that may be launched in vacant slots in such bands. The inventive method is also applicable to future non-geostationary satellites operating with linearly polarized downlinks.
In connection with the inventive satellite-based communications system, uplink signals can be provided to dedicated paired transponders on geostationary satellites. The uplink signals can contain broadcast data and correction factors that maintain a high degree of purity in the quality of the circular polarization of the signal received on the ground. The data can be broadcast (multicast) video and audio content or Internet data including large files where high-speed downlink transfer is critical. The correction factors can be optimized for each type of content based on the known location of each mobile terminal, whose location coordinates have been previously transmitted to the uplink facility.
Control circuitry at the uplink facility can optimize each transponder for either broadcast (multicast) services over a wide geographical area, or optimize each transponder for Internet data delivery (one-to-one) or local video broadcasts intended for a mobile user in a particular location. This optimization can include the ability to adjust on a packet-by-packet basis the uplink waveform based on measurements performed at the uplink or remote sites.
A second satellite can be positioned in the same orbital position to provide redundancy or double the capacity by transmitting the orthogonal circular polarization. Additional system capacity can be added on a transponder-by-transponder basis, using transponders all contained within one satellite or by using a plurality of satellites in the network.
One aspect of the present invention results in a mobile antenna with  less than xc2xc the area of prior art (for a specific satellite transponder configuration), with xcx9cxc2xd the corresponding data throughput. A spread spectrum waveform can be added to the downlink waveform, resulting in further reduction in antenna size. In this manner, a mobile terminal can employ a much smaller antenna aperture (less than xc2xc the area) than current systems, while allowing operation with one received polarization in the mobile antenna. For example, a low-cost phased array antenna can be used as a receiving antenna for a mobile unit or terminal. Mobile tracking antennas designed according to the teachings of the present invention can be implemented with electronic beam steering that contain less than xe2x85x9 the number of required elements of prior art designs.